This Phase I study will demonstrate the feasibility of combining a catheter-based automated thermodilution cardiac output technique with a truly continuous measurement of flow and velocity to provide accurate quantification of real-time beat-to-beat stroke volume. A catheter/ instrument system will be developed to continuously acquire instantaneous velocity waveforms in the pulmonary artery. These waveforms, when integrated over a cardiac cycle, are proportional to forward stroke volume. The system will be periodically calibrated in situ by using a newly-developed continuous thermodilution cardiac output measurement system to correctly scale the instantaneous velocity waveforms over a given measurement interval so as to produce instantaneous flow waveforms. Cardiac output is the fundamental performance measure for assessing the status of the heart. In the USA, the 1.3 million Swan-Ganz bolus thermodilution catheters sold annually are being progressively replaced with automated thermodilution catheter systems now entering the market. The proposed low cost catheter -instrument system has the potential for making a significant contribution to cardiac critical care cost containment by providing the stroke volume measurement at a small fraction of the cost now routinely required when the same is sought via one of the expensive imaging modalities in the management of unstable cardiac patients. There is recognized need for more sensitive and timely hemodynamic monitoring of critically ill patients. With growing acceptance of automated, quasi-continuous cardiac output monitoring, the proposed integrated continuous cardiac output / stroke volume catheter should find wide commercial potential use in critical care units, cardiac catheterization laboratories, cardiovascular experimental labs, electrophysiology testing labs, etc. PROPOSED COMMERCIAL APPLICATION: There are currently 2 million Swan-Ganz thermodilution cardiac output catheters sold world-wide (1.3 million US). There is recognized need for more sensitive and timely hemodynamic monitoring of critically ill patients. With growing acceptance of automated, quasi-continuous cardiac output monitoring, the proposed integrated continuous cardiac output / stroke volume catheter should find wide commercial potential use in critical care units, cardiac catheterization laboratories, cardiovascular experimental labs, electrophysiology testing labs, etc.